Functional Magnetic Resonance Imaging (fMRI) is used widely to study the healthy brain as well as many neuropsychiatric disorders. fMRI is a modality based on the detection of the Blood Oxygenation Level Dependence (BOLD) Effect. The BOLD signal measures variations of the transversal relaxation time susceptible to local magnetic field inhomogeneity (T2*). It is well known that neuronal activities require energy, which is achieved by consuming glucose; this process requires oxygen (O2). The mechanisms involved in transporting O2 originate the BOLD signal. The magnetic properties of these molecules modify the local magnetization of the blood’s proton content by changing its homogeneity, therefore its T2* and the BOLD signal.
The fMRI studies can be task-related, where the subjects are instructed to perform a cognitive task while MR images are acquired, or use resting state fMRI (rs-fMRI), where the subjects are instructed to stay still and relaxed during image acquisition.
Ultra-short Echo Time (UTE) Imaging
Conventional MRI methods usually target protons of unbound water that can move relatively freely in larger compartments like cells or extracellular spaces and, hence, possess a rather long transversal relaxation time (T2). Often times, however, important structural features as in collagenated tissue of the musculoskeletal system such as cortical bone, cartilage and tendons contain tissue with encapsulated water molecules that possess T2* values of a few milliseconds or less. Imaging based on these short T2 protons can be very insightful, however, they do not significantly contribute to the MR signal of standard acquisitions.
MRI Techniques that target short T2 protons are classified as zero and ultra-short echo time (ZTE and UTE) imaging as well as sweep imaging with Fourier transformation (SWIFT). Our group recently started to work on UTE imaging to develop new sequences that enhance the performance of these methods.
Diffusion Tensor Imaging (DTI)
Diffusion tensor imaging (DTI) is a diffusion-weighted MRI technique that measures the rate and direction of water diffusion. Diffusion properties of in vivo tissue strongly depend on local biological microstructures. DTI is frequently used to investigate myelinated fiber tracts in the brain. Due to myelin sheathing brain White Matter (WM) exhibits highly anisotropic diffusion patterns (preferentially along the axon bundle’s main direction) compared to Grey Matter (GM) and Cerebrospinal Fluid (CSF). DTI exploits this difference to carve out WM fiber tracts by calculating the three unit eigenvectors/eigenvalues of the molecules’ ellipsoidal movement at each voxel. DTI reconstruction leads to various applications such as fractional anisotropy (FA) maps and fiber tractography that provide insight into the WM fiber’s integrity and structure in very high detail.
The term body MRI is meant to refer to imaging parts of the rump such as pelvis and thorax and thereby includes imaging of many organs like liver, heart, lungs, and kidneys and is of very broad interest. Depending on the specific question at hand different imaging protocols are applied. However, a mutual challenge associated with body imaging is motion that occurs during the scan. The inflation and deflation of the lungs during continuous breathing leads to relative large motion of the pelvis and thorax and, therefore, causes difficulties for the acquisition of quality MR images.
Another interesting aspect of body imaging is the separation of signal deriving from fat and water tissue. Valuable information on the body fat distribution as well as body water/fat fractions can be derived from it.
The development and improvement of breath-hold and free-breathing methods that are capable to separate water and fat signal is a currently being investigated in our group.